1. Field of the Invention
The present invention relates to an FMX stereophonic receiver and, more particularly, to an FMX stereophonic receiver able to prevent deterioration of stereophonic channel separation caused by phase shifting.
2. Description of the Prior Art
FMX stereophonic broadcasting has been proposed as one means of enlarging the service area and improving the signal-to-noise ratio characteristics of FM stereo broadcasts. The transmission signal of the aforementioned FMX stereo broadcast includes a compressed stereo difference signal (L-R)' broadcast simultaneously with the transmission signal of conventional FM stereo broadcasting, for example, a stereo sum signal (L+R) and stereo difference signal (L-R). The transmission signal can be expressed as: EQU f(t)=(L+R)+Psin(w/2)t+(L-R)sinwt+(L-R)'coswt (1)
where L+R is a stereo sum signal, L-R is a stereo difference signal, P is a stereo pilot signal, and w is the subcarrier angular frequency. As shown by aforementioned Equation (1), compressed stereo difference signal (L-R)' is quadrature modulated from uncompressed stereo difference signal (L-R), resulting in an FMX stereo broadcast transmission signal spectrum shown in FIG. 1.
Furthermore, the relationship between the uncompressed stereo difference signal (L-R) and the compressed stereo difference signal (L-R)' is as shown in FIG. 2 which expresses the compression characteristics. In FIG. 2, when the input signal level is low, the aforementioned signal (L-R)' is 20 dB greater than the uncompressed stereo difference signal (L-R) and, at the same time, input/output characteristics become linear, and also the compression ratio becomes 1:1. When the level of the input signal is medium (approximately -30 dB), the compression ratio becomes .infin.:1, and input/output characteristics are flat over a range of approximately 10 dB. When the input signal level becomes high, the aforementioned signal (L-R)' rapidly attenuates. Therefore, compressed stereo difference signal (L-R)' is as shown by solid line B in FIG. 2 with respect to stereo difference signal (L-R) (solid line A), and the sum signal of the aforementioned signal (L-R) and the aforementioned signal (L-R)' is as shown by dotted line C in FIG. 2.
As discussed above, the transmission signal for FMX stereophonic broadcasting is received by a receiver as shown in FIG. 3. In FIG. 3, the FMX stereophonic broadcast transmission signal received by antenna 1 is received by a receiving circuit 2 of the same construction as a conventional FM stereophonic receiver in which stereo sum signal (L+R) (hereafter referred to as M), stereo difference signal (L-R) (hereafter referred to as S), and compressed stereo difference signal (L-R)' (hereafter referred to as S') are each demodulated. When the received signal is detected by the FM detection circuit included in the receiving circuit, stereo sum signal M is demodulated. When the stereo composite signal is detected by the synchronous detection using the 38-kHz subcarrier signal obtained from the PLL in the receiving circuit, uncompressed stereo difference signal S is demodulated. And when the stereo composite signal is detected by the quadrature detection, compressed stereo difference signal S' is demodulated.
Uncompressed and compressed stereo difference signals S and S' obtained from receiving circuit 2 are added by adder 3, and the result is applied to VCA (voltage control amplifier) 4 operating as an attenuator. When stereo difference signal S and output signal (S+S') of VCA 4 are greater than a specified level (a knee-point level), first and second level detection circuits 5 and 6, each having a threshold level, operate in such a manner that the level of stereo difference signal S and the level of aforementioned output signal (S+S') of VCA 4 are respectively detected by first and second level detection circuits 5 and 6, and are compared by comparator circuit 7. Next, a signal according to the level difference obtained from aforementioned comparator circuit 7 is rectified and smoothed by rectifying circuit 8, and the rectified signal is applied to VCA 4 as a control signal. The output signal (S+S') of aforementioned VCA 4 is controlled by this control signal to be equal to the level of stereo difference signal S. However, when aforementioned stereo difference signal S and output signal (S+S') of VCA 4 are below the knee-point level, first and second level detection circuits 5 and 6 do not operate, and attenuation at VCA 4 is fixed at approximately 20 dB.
Although stereo sum signal M obtained from receiving circuit 2 is applied directly to matrix circuit 9, stereo difference signal S or output signal (S+S') of VCA 4 are selected by switch 10, and applied to matrix circuit 9. Although not given in the above description, a 10-Hz ID signal is included in the FMX stereophonic broadcast transmission signal, and FMX stereophonic broadcasts are differentiated from conventional FM stereophonic broadcasts by the aforementioned ID signal. In addition, because a detection circuit which detects the aforementioned ID signal is built in to receiving circuit 2, whether the broadcast is FMX stereo or not can be determined with the output signal of the aforementioned detection circuit. Switch 10 is controlled by the aforementioned ID signal. When the ID signal is present, switch 10 is switched to a position as shown in FIG. 3. Accordingly, stereo sum signal M and output signal (S+S') from level controlled VCA 4 are matrixed, and left and right stereo signals L and R are generated at left and right output terminals 11 and 12. Furthermore, when the ID signal is not present, switch 10 is switched to a position opposite to that shown in FIG. 3, and stereo sum signal M and stereo difference signal S are matrixed in matrix circuit 9.
As described above, because FMX stereophonic broadcast system uses compressed and expanded stereo difference signal S, it is possible to achieve significant improvements in the S/N ratio, and the service area can be enlarged comparably equal to that of the conventional monaural FM broadcast system.
It is to be noted that the FMX stereophonic broadcast transmission signal can be accurately received by a conventional FM stereophonic receiver. In this case, compressed stereo difference signal S' is quadrature modulated with respect to stereo difference signal S, and reception is not adversely affected.
Details concerning FMX stereophonic broadcasting are disclosed, for example, in an article "Improving the Signal-to-Noise Ratio and Coverage of FM Stereophonic Broadcasts" by Emil L. Torick and Thomas B. Keller in "JOURNAL OF THE RADIO ENGINEERING SOCIETY", volume 33, number 12, issued December 1985.
Because FMX stereophonic broadcasting is currently in the experimental stage, and there is no current broadcasting available, FMX stereophonic receivers are, of course, not commercially available. However, when a receiver in FIG. 3 was actually designed and experimented with a test signal to measure the characteristics, channel separation during FMX stereophonic broadcast reception was found to deteriorate. Specifically, uncompressed stereo difference signal S and compressed stereo difference signal S' are synchronously detected and quadrature detected, respectively, using a signal obtained from a PLL circuit locked to a 19-kHz pilot signal included in an FM detection output signal. However, if the phase relationship between the phases of the detection signal and the signal to be detected undesirably deviates due to the phase characteristics of a frequency divider provided in a PLL circuit, or due to an offset of the phase comparator provided in the PLL circuit, normal detection output cannot be obtained. Accordingly, when output signal (S+S') of VCA 4 and stereo sum signal M are matrixed, separation can not be accomplished in a desired form. A 38-kHz detection signal obtained from the PLL circuit may generally cause a phase shift of about 10 degrees with respect to the 38-kHz subcarrier, but in FMX broadcast system, such a phase shift will result in a great deterioration of the channel separation, because in FMX broadcast system, a phase shift of 2 degrees will result in channel separation deterioration by over 10 dB.
Furthermore, stereo sum signal M, uncompressed stereo difference signal S, and compressed stereo difference signal S' obtained from the receiving circuit each contain a high harmonic frequency component. In a standard FM stereophonic receiver, there are no particular problems even when the signals are matrixed containing the high frequency component. Nevertheless, in the FMX stereophonic receiver, because the level of output signal (S+S') of VCA 4 is controlled according to the level of uncompressed stereo difference signal S, if the high frequency component is applied to the first level detection circuit 5, which detects the uncompressed stereo difference signal S level, the level of the output signal (S+S') of VCA 4 changes, and stereo separation during FMX stereo broadcast reception deteriorates.
Moreover, in the circuit of FIG. 3, there is the additional problem of the circuit becoming complex because two level detection circuits are required, and it is therefore necessary to match the characteristics of both level detection circuits.